The radius around an aggro monster in a MMORPG game whereby if a player enters this radius, the monster will attack the player. Knowing the aggro range of the monsters in a game can be the key to staying alive in dangerous areas.
-Player1: "Be careful those guys have a huge aggro range."
-Player2: "I know."
-Player1: "That's too close, get back!"
-Player2: "Oh no! Aggro!"
-Player1: "Jeeze, what a n00b."